Selector circuit for power management in multiple battery systems

ABSTRACT

A selector circuit configured to select among a DC power source and a plurality of batteries for an electronic device. The selector circuit is responsive to an output signal from an associated power management unit. The selector circuit is further configured to permit parallel operation of two or more of the batteries. The selector circuit may further act to independently verify power conditions and override instructions from the PMU in certain instances to enhance power supply safety and battery life such as by preventing inter battery current flow from a higher potential battery to a lower potential battery coupled in parallel. A power supply block including the selector circuit and an electronic device including the power supply block is also provided. The selector circuit may be integrated with a charging circuit on one integrated circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part application ofU.S. Nonprovisional application Ser. No. 10/364,228 filed Feb. 11, 2003,the teachings of which are incorporated herein by reference and claimsthe benefit of U.S. Provisional Application No. 60/484,635 filed Jul. 3,2003, the teachings of which are also incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to selector circuits and inparticular to selector circuits for use with multiple battery systems.

BACKGROUND OF THE INVENTION

[0003] Selector circuits are typically utilized in a power supply blockfor various electronic devices. Such selector circuits are generallydesigned to select between a DC power source, e.g., an AC/DC adapter,and a rechargeable battery. In addition, in various electronic deviceslike a laptop computer, such selector circuits are typically controlledvia control signals communicated via a System Management Bus (SMBus)according to a specified protocol. In addition, such selector circuitstypically cannot independently ascertain, correct, and notify othercomponents in the power supply block of a power crises condition. Inaddition, such selector circuits are not configured to accept controlsignals from an associated host power management unit.

[0004] Accordingly, there is a need in the art for a selector circuitfor overcoming the above deficiencies in the prior art.

BRIEF SUMMARY OF THE INVENTION

[0005] In one embodiment, a power supply system consistent with theinvention includes: a first path configured to be coupled to a firstbattery; a second path configured to be coupled to a second battery, thefirst path and the second path coupled to a common node; a first switchand a second switch coupled to the first path and configured to allowselective coupling of the first battery to a load for discharging thefirst battery; a third switch and a fourth switch coupled to the secondpath and configured to allow selective coupling of the second battery tothe load for discharging the second battery; and a selector circuitconfigured to close the first, second, third, and fourth switch to allowdischarging of the first battery and the second battery in parallel if afirst discharge current level from the first battery is greater than afirst discharge threshold level and a second discharge current levelfrom the second battery is greater than a second discharge thresholdlevel.

[0006] In another embodiment, an electronic device is provided. Theelectronic device includes: a first path configured to be coupled to afirst battery; a second path configured to be coupled to a secondbattery, the first path and the second path coupled to a common node; afirst switch and a second switch coupled to the first path andconfigured to allow selective coupling of the first battery to a systemof the electronic device for discharging the first battery; a thirdswitch and a fourth switch coupled to the second path and configured toallow selective coupling of the second battery to the system fordischarging the second battery; and a selector circuit configured toclose the first, second, third, and fourth switch to allow dischargingof the first battery and the second battery in parallel if a firstdischarge current level from the first battery is greater than a firstdischarge threshold level and a second discharge current level from thesecond battery is greater than a second discharge threshold level.

[0007] In yet another embodiment consistent with the invention, a methodof ensuring safe operation of batteries in parallel is provided. Themethod includes: receiving a control signal from an associated powermanagement unit representative of a desired parallel coupling of atleast a first battery and a second battery to a common node; receiving afirst current signal representative of a first current level along afirst path coupled between the first battery and the common node;receiving a second current signal representative of a second currentlevel along a second path coupled between the second battery and thecommon node; comparing the first current signal to a first thresholdcurrent level and the second current signal to a second thresholdcurrent level; and coupling the first battery and second battery inparallel to the common node if the first current signal is greater thanthe first threshold current level and the second current signal isgreater than the second threshold current level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Advantages of the present invention will be apparent from thefollowing detailed description of exemplary embodiments thereof, whichdescription should be considered in conjunction with the accompanyingdrawings, in which:

[0009]FIG. 1 is a simplified high level block diagram of an electronicdevice with a power supply block having a selector circuit consistentwith the invention that makes a selection in response to an outputsignal from a power management unit (PMU);

[0010]FIG. 2 is a more detailed block diagram of the power supply blockportion of FIG. 1 having a selector circuit consistent with theinvention for making a selection among a DC power source and a pluralityof batteries;

[0011]FIG. 3 is a block diagram of one exemplary embodiment of aselector circuit consistent with the invention having a controllerconfigured to provide signals to select among a DC power source and aplurality of batteries via an associated switch driver network andassociated switches;

[0012]FIG. 4 is a more detailed block diagram of the selector circuit ofFIG. 3 illustrating various components of the controller portion in moredetail;

[0013]FIG. 5 is an exemplary table illustrating how the selector circuitdrives various switches to ON and OFF states dependent on various inputsignals when the electronic device is powered by a DC power source;

[0014]FIG. 6 is an exemplary table illustrating how the selector circuitdrives various switches to ON and OFF states depending on various inputsignals when the device is powered by various combinations of batteries;

[0015]FIG. 7 is a block diagram of another exemplary embodiment of aselector circuit consistent with the invention having a controllerconfigured to provide signals to select among a DC power source and aplurality of batteries via an associated switch driver network andassociated switches;

[0016]FIG. 8 is a more detailed block diagram of the selector circuit ofFIG. 7 illustrating various components of the controller portion in moredetail;

[0017]FIG. 9 is an exemplary table illustrating how the selector circuitof FIG. 7 drives various switches to ON and OFF states dependent onvarious input signals when the electronic device is powered by a DCpower source;

[0018]FIG. 10 is an exemplary table illustrating how the selectorcircuit of FIG. 7 drives various switches to ON and OFF states dependingon various input signals when the device is powered by variouscombinations of batteries; and

[0019]FIGS. 11A to 11C are exemplary circuit diagrams illustrating howthe selector circuit of FIG. 7 detects a low voltage battery conditionand prevents inter battery current flow.

DETAILED DESCRIPTION

[0020] Turning to FIG. 1, a simplified block diagram of an electronicdevice 100 capable of being powered from any number of power sources104, 105 is illustrated. Such power sources may include a plurality ofbatteries 105 and a DC power source 104. The batteries 105 may furtherbe rechargeable batteries of various types such as lithium-ion,nickel-cadmium, nickel-metal hydride batteries, or the like. Theelectronic device 100 may be any variety of devices known in the artsuch as portable electronic devices (laptop computers, cell phones,pagers, personal digital assistants, camcorders, digital cameras, radiocassette players, and the like), an electric powered vehicle, powertools, etc. that may be powered from either power source 104, 105 invarious instances.

[0021] If the electronic device 100 is a laptop computer it wouldinclude a variety of components known to those skilled in the art whichare not illustrated in FIG. 1. For example, the laptop may include aninput device for inputting data to the laptop, a central processing unit(CPU) or processor, for example a Pentium processor available from IntelCorporation, for executing instructions and controlling operation of thelaptop, and an output device, e.g., a LCD or speakers, for outputtingdata from the laptop.

[0022] To recharge batteries 105 and/or supply power to the device 100,a DC power source 104 may be coupled to the device 100. The DC powersource 104 may be an AC/DC adapter which is configured to receiveconventional 120 volts AC from a wall outlet and convert it to a DCoutput voltage. The DC power source 104 may also be a DC/DC adapter suchas a “cigarette lighter” type adapter configured to plug into that typeof socket. Such a DC power source 104 is illustrated in FIG. 1 asseparate from the device 100, but it may be built into some devices.

[0023] The device 100 has a power supply block 106 including at least aselector circuit 114 consistent with the present invention. The powersupply block 106 may also include a PMU 120 as illustrated in FIG. 1.Alternatively, the PMU 120 may also be embedded in a more complexprocessor of the electronic device 100. The PMU 120 is configured to runvarious power management routines as is known in the art. In general,the power supply block 106 includes various components to monitor,control, and direct power from each power source to each other and tothe system 110 of the device 100 under various conditions.Advantageously, the selector circuit 114 consistent with the inventionis configured to be responsive to at least one output signal from thePMU 120 as further detailed herein.

[0024] Turning to FIG. 2, a more detailed block diagram of an exemplarypower supply block 206 for a multiple battery system is illustrated. Thepower sources may include the DC source 204, e.g., an AC/DC converter,and any number of a plurality of batteries 205-1, 205-2, 205-k. Suchbatteries may also be rechargeable batteries. At any point in time, eachof these power sources 204, 205-1, 205-2, 205-k may or may not bepresent in the system.

[0025] In general, the power supply block 206 may include a PMU 220, acharger circuit 222, a power conversion element 226, a battery switchnetwork 217, a switch 230, a power supply path 209 from the DC powersource 204 to the system 210, a power supply path 240 from the batteries205-1, 205-2, 205-k to the system, a power supply path 207 from the DCpower source 204 to the rechargeable batteries 205-1, 205-2, 205-k forrecharging purposes, a selector circuit 214 consistent with theinvention, and various data or communication paths. The battery switchnetwork 217 may further contain a charge switch CSW1, CSW2, CSWk and adischarge switch DSW1, DSW2, DSWk for each associated battery 205-1,205-2, 205-k.

[0026] The data or communication paths between the various components ofthe power supply block 206 may be uni-directional or bi-directional, andmay conduct either analog or digital signals. The data paths maytransport either command or control signals or data. The number of suchdata paths is strongly dependent on the particular features of thebatteries 205-1, 205-2, 205-k, the charger circuit 222, the PMU 220, andthose of the supply block 206 as a whole. For example, if an associateddevice 100 is a laptop computer, a smart charger circuit and smartbatteries can communicate via a System Management Bus (SMBus) accordingto a specified protocol.

[0027] In general, the selector circuit 214 is responsive to variousinput signals from a variety of components, including the PMU 220, inthe supply block 206 to provide switch control signals over path 250 tothe battery switch network 217 and the switch 230 to control and directpower from each power source to each other and to the system 210 undervarious conditions.

[0028] For example, a particular set of input signals to the selectorcircuit 214 may indicate the presence of a DC power source 204 with anacceptable voltage level. In response to such an input signal, theselector circuit 214 could provide a control signal to switch 230 toclose (turn ON) switch 2300N and to open (turn OFF) discharge switchesDSW1, DSW2, DSWk in the battery switch network 217. As such, power fromthe DC power source 204 would be provided to the system 210.Alternatively, if input signals to the selector circuit indicated theabsence of a DC power source 204 or a DC power source with anunacceptable voltage level, the selector circuit 214 would provide anappropriate control signal to turn switch 230 OFF, and to turn one ofthe discharging switches DSW1, DSW2, DSWk of the battery switch networkON. As such, one or more of the associated batteries 205-1, 205-2, 205-kwould provide power to the system 210 as long as other safety conditionswere also met as will be further detailed herein.

[0029] The charge switches CSW1, CSW2, CSWk for each associatedrechargeable battery 205-1, 205-2, 205-k provide a conductive path fromthe power supply line 207 to each associated battery when the chargeswitches are ON for charging purposes. The discharge switches DSW1,DSW2, DSWk provide a conductive path from each associated battery 205-1,205-2, 205-k to the system 210 to power the system 210 from one or morebatteries based on which discharge switches DSW1, DSW2, DSWk are ON.

[0030] Advantageously, at least one input signal to the selector circuit214 is representative of an output signal from the PMU 220. Suchcommunication between the PMU 220 and the selector circuit 214 may takeplace via data path 211. As understood by those skilled in the art, thePMU 220 is capable of running a host device's power management routine.The PMU 220 may provide a host set of signals to the selector circuit214 including a signal indicating which battery 205-1, 205-2, 205-k, orcombination of batteries in parallel, should be selected for charging ordischarging. As further detailed herein, the selector circuit 214 isresponsive to the PMU 220. However, the selector circuit 220 is furtherconfigured to have its own internal checks and can override a desireduse signal from the PMU under various conditions as further detailedherein to provide for added safety and battery power savings. Thecharger circuit 222 is configured to communicate via data path 252 tothe selector 214 and via data path 254 to a power conversion unit 226,e.g., a charger controlled DC-DC converter. The charger circuit 222 maycontrol the providing of charging current to the batteries 205-1, 205-2,205-k via the power supply path 207 and the power conversion unit 226.

[0031] Turning to FIG. 3, an exemplary power supply block 306 foroperation in conjunction with three power sources is illustrated. Thepower sources include a DC power source (not illustrated) coupled to thepower supply block 306 via power supply path 309, a first rechargeableBattery A, and a second rechargeable Battery B. The power supply block306 includes a selector circuit 314 consistent with the invention andother components such as an associated PMU 320, a charger circuit 322,and a power conversion unit 326, e.g., a DC-DC converter. As earlierdetailed, although the PMU 320 is illustrated as part of the supplyblock 306, the PMU 320 may be external to the supply block, embedded ina separate component outside of the power supply block, or the PMU'sfunctionality may be provided by a separate component, e.g., a CPU, ofthe electronic device.

[0032] For clarity and simplicity, the DC source and various dataconnections (e.g., from the charger circuit 306 to the power conversionunit 326 and to the PMU 320, as well as those between the batteries andthe PMU 320) that were previously illustrated in FIG. 2 are notillustrated in FIG. 3. Advantageously, the selector circuit 314 and thecharging circuit 322 may be integrated onto one integrated circuit 390for convenience of operation and installation.

[0033] The selector circuit 314 includes a controller 315 and a switchdriver network 317 as further detailed herein. The selector circuit 314has a variety of input terminals 380 to accept a variety of input dataand control signals. Such input terminals 380 are also coupled to thecontroller 315. The selector circuit 314 also has a variety of outputterminals 382 to provide control signals to associated switches SW1,SW2, SW3, SW4, SW5, and SW6 and to provide data to associated componentsof the power supply block 306. The input terminals 380 include terminals380-1 to 380-9 to accept control and data signals labeled PSM, USE_A,USE_B, ICHG, VAD, VSYS, BATT_A, BATT-B, and AUXIN respectively. Theoutput terminals 382 include terminals 382-1 to 382-10 to providecontrol and data signals labeled PWR_AC, PWR_BATT, CHGA, DCHA, ACAV,ALERT, CHGEN, CHGB, DCHB, and AUXOUT respectively. Each input terminal380 and output terminal 382 and their associated control and datasignals are generically described below.

[0034] The first input terminal 380-1 may accept a power save mode (PSM)digital input control signal from the PMU 320 representative of whethera power save mode is desired by the PMU 320. The second and third inputterminals 380-2 and 380-3 may accept USE_A and USE_B control signalsfrom the PMU 320 indicating the PMU's desired battery or combination ofbatteries to utilize in a given charging or discharging mode. Forinstance, in the embodiment of FIG. 3 having two batteries A and B theUSE_A and USE_B control signals may be digital signals such that ifUSE_A is low and USE_B is high, use of Battery A is desired. If USE_A ishigh and USE_B is low, use of Battery B is desired. If USE_A is low andUSE_B is low, use of Battery A and Battery B in parallel is desired.Finally, if USE_A is high and USE-B is high, use of neither Battery Anor Battery B is desired. These representative high and low signals forUSE_A and USE_B is for illustrative purposes only as those skilled inthe art will recognize that other combinations may also be chosen.

[0035] The fourth input terminal 380-4 may accept a charging current(ICHG) analog signal from the charger circuit 322 representative of thecharging current provided to the batteries. The fifth input terminal380-5 may accept an analog signal from the DC voltage source 204, e.g.,the AC/DC adapter, (VAD) representative of the voltage level provided bythe DC power source 204 at that particular time. The sixth inputterminal 380-6 may accept an analog signal representative of the systemsupply voltage level (VSYS). The seventh 380-7 and eighth inputterminals 380-8 may accept analog signals from Battery A (BATT_A) andBattery B (BATT_B) representative of the voltage level of eachrespective battery. Such BATT_A and BATT_B analog signals may beobtained by measuring the voltage at the positive pole of eachrespective battery. Finally, the ninth input terminal 380-9 represents ageneric input terminal capable of receiving any other input control anddata signals (AUXIN) considered not critical to the description of thepresent invention herein.

[0036] The first output terminal 382-1 may provide a switch controlsignal (PWR_AC) to switch SW1. The second output terminal 382-2 mayprovide a switch control signal (PWR_BATT) to switch SW2. The thirdoutput terminal 382-3 may provide a switch control signal (CHGA) to thecharging switch SW3 for Battery A. The fourth output terminal 382-4 mayprovide a switch control signal (DCHA) to the discharging switch SW4 forBattery A. The fifth output terminal 382-5 may provide a digital DCsource enable signal (ACAV) indicating the presence or absence of the DCPower source 204 having an output voltage greater than an acceptablethreshold limit.

[0037] The sixth output terminal 382-6 may provide a digital data signal(ALERT) to notify other components, including at least the PMU 320, of apower crisis condition which will be later detailed herein. The seventhoutput terminal 382-7 may provide a digital data signal (CHGEN) to thecharger which indicates if a charge enable condition has been reached.The eighth output terminal 382-8 may provide a switch control signal(CHGB) to the charging switch SW5 for Battery B. The ninth outputterminal 382-9 may provide a switch control signal (DCHB) to thedischarging switch SW6 for Battery B. Finally, the tenth output terminal380-10 represents a generic output terminal capable of providing anyother output control and data signals (AUXOUT) considered not criticalto the description of the present invention herein.

[0038] The controller 315 accepts the above input data and controlsignals from the input terminals 380 of the selector circuit 314 andmakes decisions about which power source or combination of sources(e.g., DC power source, Battery A, or Battery B) to select or deselectby controlling one or more combinations of switches SW1 to SW6. Thecontroller 315 may also provides data and other control signals directlyto the other output terminals, e.g, output terminals 382-5, 382-6,382-7, and 382-10, for communication to other components of the powersupply block 306.

[0039] The switch driver network 317 may include a plurality of switchdrivers SD1, SD2, SD3, SD4, SD5, and SD6. Each of the switch driversSD1, SD2, SD3, SD4, SD5, and SD6 may be further coupled to an associatedswitch SW1, SW2, SW3, SW4, SW5, and SW6 in order to drive each switch toON and OFF positions as instructed by the controller 315 of the selectorcircuit 314.

[0040] Turning to FIG. 4, a more detailed block diagram of the selectorcircuit 314, and in particular the controller 315 of the selectorcircuit 314 of FIG. 3 is illustrated. In general, the controller 315 mayinclude a selector output circuit 470, a charge enable circuit 472, aparallel battery use enable circuit 476, an input validation circuit478, a power crises circuit 474, and a plurality of comparators CMP1,CMP2, CMP3, and CMP4.

[0041] In general, the selector output circuit 470 may receive a varietyof internal control signals such as a charge enable (CHGEN) signal fromthe charge enable circuit 472, a diode mode (DM) signal from the powercrises circuit 474, a valid input signal (VINP1) from the inputvalidation circuit 478, a parallel battery use enable (PBUE) signal fromthe parallel battery use enable circuit 476, and a DC source enablesignal (ACAV) from comparator CMP1. The selector output circuit 470 mayalso receive an analog signal ICHG from the charger circuit 322representative of the charging current. As further detailed herein, theselector output circuit 470 directs the switch driver network 317 toturn associated switches SW1, SW2, SW3, SW4, SW5, and SW6 ON and OFFdepending on the state of various input signals.

[0042] The controller 315 may include a first comparator CMP1 configuredto compare an analog signal representative of the voltage level of theDC source with a first threshold level VT1. The first threshold levelVT1 is set higher than minimum supply voltage VT3 acceptable to thesystem. If the DC power source is present and has a supply voltagegreater than the first threshold level VT1, the first comparator CMP1provides a high ACAV control signal to the selector output circuit 470.Otherwise the first comparator provides a low ACAV signal The ACAVsignal may also be provided to the power crisis circuit 474.

[0043] If the selector output circuit 470 receives a high ACAV signalfrom the first comparator CMP1, it will provide appropriate switchcontrol signals to turn switch SW1 ON and turn switches SW2 to SW6 OFF(assuming the DC power source supply voltage is not greater than asecond threshold level VT2 as further detailed below) such that power tothe system 210 will be provided by the DC power source and no batterieswill be recharged. The selector circuit 314 will utilize the DC powersource in this instance irrespective of the USE_A and USE_B controlsignals from the PMU. As such, the selector circuit 314 can override acontrol signal from the PMU to use Battery A or Battery B and insteadrequire power to the system 210 to be supplied by the DC power sourcewhenever it is present and has a suitable voltage level greater thanVT1. Advantageously, this feature prolongs battery life by ensuring useof the DC power source in appropriate circumstances.

[0044] To enable powering of the system 210 from the DC source andcharging of one or more batteries, the charge enable (CHGEN) signal mustbe active. An active CHGEN signal in the present embodiment is a highCHGEN signal. The charge enable circuit 472 will provide a high CHGENsignal if it receives an appropriate CHGP signal from the secondcomparator CMP2, and an appropriate validation signal VINP1 from theinput validation circuit 478. The second comparator CMP2 provides theappropriate CHGP signal if the supply voltage from the DC power sourceis greater than a second threshold level VT2, where VT2>VT1, andVT1>VT3. The input validation circuit 478 provides the validation signalVINP1. An appropriate validation signal VINP1 will be provided if theUSE_A and USE_B control signals from the PMU assert the use of at leastone of the Batteries A or B. An appropriate validation signal VINP1 willnot be sent if the USE_A and USE_B control signals fail to assert theuse of any of the Batteries A or B, e.g., if both USE_A and USE_B arehigh. The charge enable circuit 472 may also need other supplementaryvalidation input signals (AUXIN) from the generic input terminal 380-9in order to generate an active CHGEN signal.

[0045] During charging, the charging circuit 322 provides the ICHGsignal to the selector circuit 314 that is representative of a chargingcurrent level. The selector circuit 314 accepts the ICHG signal at inputterminal 380-4 and provides such signal to the selector output circuit470. The selector output circuit 470 compares such ICHG signal with acharging threshold level signal ICHT. Based on this comparison, theselector output circuit 470 decides if the charging current level ishigh or low and turns various switches ON or OFF based on this and otherinput data as further detailed herein. A low charging current isrepresented by a low control signal and a high charging current isrepresented by a high control signal in the present embodiment asdetailed in the table of FIG. 5.

[0046] The parallel battery use enable circuit 476 provides a parallelbattery use enable (PBUE) signal to the selector output circuit 470. Theselector output circuit 470 responds to a high PBUE signal by allowingparallel battery use, and responds to a low PBUE signal by not allowingparallel battery use despite a request from the PMU 320 via USE_A andUSE_B signals indicating a desire for parallel battery use, e.g., USE_Aand USE_B are low. As such, the selector circuit 314 provides additionalprecautions and protections against using Batteries A and B in parallelunless appropriate conditions are present.

[0047] For instance, the concern with using any two or more batteries,e.g., Battery A and Battery B, in parallel is that there is a relativelylarge difference in potential that creates an undesirable high currentconditions when such batteries are connected in parallel. As such, afourth comparator CMP4 of the controller 315 is configured to comparesignals BATT_A and BATT_B. Such BATT_A and BATT_B signals may be analogsignals taken from the positive terminal of Battery A and Battery B. Ifthe difference between the two BATT_A and BATT_B signals is within apredefined limit, the comparator CMP4 will provide an active BATTCOMPsignal to the parallel battery use enable circuit 476. In addition toreceiving an active BATTCOMP signal from the fourth comparator CMP4, theparallel battery use enable circuit 476 should also receive anappropriate input validation signal VINP2 from the input validationcircuit 478 to issue an active PBUE signal. An appropriate validationsignal VINP2 will be provided if the USE_A and USE_B control signalsassert the use of at the Batteries A and B in parallel, e.g., USE_A andUSE_B are low.

[0048] If the USE_A and USE_B control signals from the PMU indicateparallel battery use is desired by the PMU, but the PBUE signal is notactive because the voltage difference between Battery A and Battery B isnot within the predetermined limit, the selector output circuit 474 willdirect charging to the battery having the lower voltage level comparedto the other. Under similar conditions, when no valid DC source ispresent, the selector output circuit will direct the battery with thehigher voltage level compared to the other to provide discharging powerto the system.

[0049] Advantageously, the selector circuit 314 may also include a powercrises circuit 474 designed to independently monitor and identify powercrises conditions, and provide an appropriate diode mode (DM) controlsignal to the selector output circuit 470 in case of a detected powercrisis condition. The selector output circuit 470 is responsive to theappropriate DM control signal from the power crises circuit 474 to causeswitch drivers from the switch driver network 317 to maintain switchesSW2, SW4, and SW6 in an ON state, while maintaining switches SW1, SW3,and SW5 in an OFF state. As such, the power source with the highestvoltage (Battery A, Battery B, or the DC power source) will supply thesystem though one of the diodes D1, D3, or D5 respectively in this diodemode. In addition, the selector circuit 314 will also provide an ALERTcondition signal at output terminal 382-6 indicating a power crisescondition. The ALERT signal could be provided to a number of components,including at least the PMU 320.

[0050] A power crises condition can include an invalid output or aninvalid input. An invalid output can occur whenever the power source orsources that are supplying the system can not maintain the systemvoltage level at the minimum system threshold voltage level VT3. Thesystem voltage level is compared with the minimum threshold voltagelevel VT3 by comparator CMP3 and a system check control signal VSYSOK issent to the power crises circuit 474 based on this comparison. A lowsystem voltage power crisis condition may occur if one or more of thepower sources are willingly or accidentally disconnected.

[0051] An invalid input can also cause a power crises problem. Aninvalid input could be the PMU asserting through USE_A and USE_B signalsa desired condition that would cause the system to lose power. Forinstance, the USE_A and USE_B signals may assert neither battery to beused (low VINP1 signal), e.g., USE_A and USE_B high, yet the DC powersource is not available (low ACAV signal) or cannot keep the system atthe minimum VT3 voltage level (low VSYSOK signal). Another invalid inputsituation may occur if the USE_A and USE_B signals from the PMU,although logically correct, would cause the system to lose power. Forinstance, the USE_A and USE_B signals may point to supply from onebattery that is not present or accidentally removed. Use of such abattery would then cause the voltage level on the system to drop belowthe VT3 threshold and the VSYSOK signal indicative of this conditionwould be provided to the power crises circuit 374.

[0052] Due to power dissipation on diodes D1, D3, or D5 it is notsuitable to maintain the DM supply mode for longer periods of time.Advantageously, the power crisis circuit 474 continuously monitors itsinput signals to deactivate is DM signal as soon as the power crisescondition is remedied. Therefore, as soon as the power crises conditionis remedied (e.g., a missing power source is coupled to the system) theinternal DM signal from the power crisis circuit becomes inactive and anormal power supply mode is resumed.

[0053] Turning to FIG. 5, in conjunction with FIGS. 2 though 4, a table500 illustrates respective switch states of switches SW1 to SW6depending on various input signals to the selector circuit 314 and theselector output circuit 470. The table 500 illustrates various switchstates when power to the system 210 is provided by the DC power source204 and not the batteries 305. As such, the ACAV signal is high and theselector output circuit 470 sends appropriate switch control signals tothe switch driver network 317 so SW1 is ON and SW2 is OFF as indicatedin every column of table 500.

[0054] The CHGEN signal is “high” in every column of the table 500except for the last column 522. As such, not only is the DC sourcepresent but the other conditions (the voltage from the DC source >VT2,and a proper input validation signal VINP1 is present) are satisfied toprovide the high CHGEN signal. As such, charging is permitted in columns502 to 520 of table 500.

[0055] In columns 502 and 504, the USE_A and USE_B signals are low andhigh respectively indicating the PMU's desire to use Battery A. As such,the switches SW5 and SW6 to Battery B are OFF in both instances. Incolumn 502, the charging current signal is “low” indicating the chargingcurrent from the power conversion unit 226 to the batteries 305 is lowerthan a threshold charging current level ICHT. As such, the selectoroutput circuit 470 is responsive to the charging current signal bysending appropriate control signals to the switch drive network 317 toturn SW3 ON and SW4 OFF. As such, charging current to Battery A flowsthrough closed SW3 and the diode D4 in parallel with open SW4. Since thecharging current is low, its flow through diode D4 will producenegligible power dissipation.

[0056] In contrast, the charging current in column 504 is high asindicated by a “high” charging current signal. As such, switches SW3 andSW4 are both ON. Therefore, no excess power is dissipated in diode D4 inthis instance since the current flows through the closed switch SW4.Normally, at similar current levels switches SW1 to SW6, when in an ONstate, dissipate less power than their corresponding parallel diodes D1to D6. This difference is particularly important at high current levels.

[0057] Turning to columns 506 and 508, the USE_A and USE_B signals arehigh and low respectively indicating the PMU's desire to use Battery B.As such, the switches SW3 and SW4 to Battery A are OFF. Column 506,somewhat similarly to column 502, has a low charging current asrepresented by the low charging current signal. As such, switch SW5 isON and SW6 is OFF. Charging current to Battery B therefore flows throughclosed switch SW5 and the diode D6 in parallel with open switch SW6. Incontrast, the charging current in column 508 is high as represented bythe high charging current signal. As such, switches SW5 and SW6 are ONsuch that no power is dissipated in diode D6 in this instance.

[0058] Turning to columns 510 to 520, the USE_A and USE_B signal are lowand low respectively indicating the PMU's desire to use Battery A andBattery B in parallel. If the parallel battery use enable (PBUE) signalis high as indicated in columns 510 and 512, parallel charging of theBatteries A and B will be permitted. Switches SW3 to SW6 will all be ONif the charging current is high (charging current signal is high) asillustrated in column 512. Switches SW3 and SW5 will be ON and switchesSW4 and SW6 will be OFF if the charging current is low (charging currentsignal is low) as illustrated in column 510.

[0059] If the USE_A and USE_B signals indicate the PMU's desire to useBattery A and Battery B in parallel, but the PBUE signal is low, theselector circuit 314 will not permit parallel battery operation therebyoverriding the PMU's desired parallel operation. With all else beingacceptable, the selector circuit 314 will permit charging of the batterywith the lower voltage level. For instance, columns 514, 516 indicateBattery A has the lower voltage level. As such, switches SW5 and SW6 toBattery B are OFF. Switches SW3 to Battery A is ON in column 510 becauseof a low charging current and switches SW3 and SW4 are ON in column 512because of a high charging current. Similarly, if Battery B has thelower voltage level, switches SW3 and SW4 to Battery A will remain OFFas illustrated in columns 518 and 520. Switches SW5 and SW6 to Battery Bwill turn ON depending on the charging current level.

[0060] In contrast to power being supplied by the DC power source, powermay be supplied by one or more of the batteries in various battery powersystem supply modes. In a battery supply mode, the selector circuit 314instructs switch SW1 to be OFF and SW2 to be ON. The selector circuit314 instructs a battery supply mode to be instituted if the DC source isnot present, or the DC is present but does not have a voltage levelabove the first threshold VT1 as determined by comparator CMP1. As such,the ACAV signal from the first comparator CMP1 to the selector outputcircuit 470 would be low indicating a battery supply mode. When the ACAVsignal is low, the selector output circuit 470 will instruct SW1 toswitch OFF and SW2 to switch ON.

[0061] In the embodiment of FIG. 3, there are essentially two normalbattery system supply modes. In normal battery system supply mode 1(nbssm1), the USE_A and USEB signals from the PMU point to use of onlyone Battery A or B, the targeted battery is present and can supply thesystem at least a voltage level to enable the system to have a voltagelevel greater the VT3 threshold level. In normal battery system supplymode 2 (nbssm2), the USE_A and USE_B signals point to the use ofBatteries A and B in parallel, both batteries are present, bothbatteries can supply the system at least a voltage level to enable thesystem to have a voltage level greater the VT3 threshold level, and bothbatteries have a respective voltage level within a predetermined voltagerange of one another.

[0062]FIG. 6 illustrates a table 600 showing various input signals forboth battery system supply modes nbssm1 and nbssm2 and the correspondingstate of switches SW1 to SW6. As indicated earlier, since battery systemsupply mode is instituted, switch SW1 is OFF and SW2 is ON. Columns 602and 604 of table 600 illustrate the first battery supply mode nbssm1where use of Battery A (column 602) or Battery B (column 604) istargeted or desired. The input validation signals VINP1 and VINP2 shouldbe at acceptable levels (VINP1 high and VINP2 low) in these instances.Therefore, if power is to be supplied by Battery A (column 602),switches SW3 and SW4 will be ON and switches SW5 and SW6 will be OFF. Incontrast, if power is to be supplied by Battery B (column 604), switchesSW5 and SW6 will be ON and switches SW3 and SW4 will be OFF.

[0063] In the second normal battery supply mode (nbssm2), BATTCOMPsignal from the comparator CMP4 is high indicating the voltages ofBatteries A and B are within an acceptable limit. The parallel batteryuse enable (PBUE) signal is also high indicating all other conditions(including high VINP2 signal) for parallel battery use as monitored bythe parallel battery use enable circuit 476 are satisfactory. As suchswitches SW3 and SW4 coupled to Battery A are ON and switches SW5 andSW6 coupled to Battery B are ON.

[0064] Somewhat similar to the charging situation, if USE_A and USE_Bsignals indicate a desire to use both Batteries A and B in parallel, butthe PBUE signal is not enabled (e.g., PBUE is low), the battery with thehigher voltage level compared to the other will be selected to providedischarging power to the system. As such, the switch states will be likethat in column 602 if Battery A has the higher voltage and like that incolumn 604 if Battery B has the higher voltage.

[0065] The PMU 320 may also send a power save mode request to theselector circuit 314 if a DC power source is absent and low powerconsumption is desired to conserve battery life. If such a power savemode request is received by the selector circuit 314, the controller 315will direct switch SW1 to turn OFF, switch SW2 to turn OFF, switch SW3to turn OFF, switch SW4 to turn ON, switch SW5 to turn OFF, and switchSW6 to turn ON. As such, Battery A or B with the higher voltage levelwill supply power via an associated diode D3 or D5 respectively. Inaddition, the selector circuits 314 own supply current will be highlyreduced compared to normal operation contributing to overall devicepower savings in this power save mode.

[0066]FIG. 7 illustrates another embodiment of a selector circuit 714consistent with the invention. The selector circuit 714 includes acontroller 715 and a switch driver network 317. In general, thecontroller 715 provides control signals to the switch driver network 317to drive the switches SW1, SW2, SW3, SW4, SW5, and SW6 ON or OFF thusselecting various power sources as further detailed herein. Like theembodiment of FIG. 3, the selector circuit 714 is configured to providefor safe operation of batteries in parallel. In general, the selectorcircuit 714 prevents coupling of batteries in parallel if undesirableconditions are present despite a parallel battery coupling request fromthe PMU 320. One undesirable condition may be one battery having agreater potential than the other battery such that undesirable interbattery current flow from the higher potential battery to the lowerpotential battery occurs.

[0067] Many elements of FIG. 7 are similar to that of FIG. 3 and, assuch, are labeled similarly. Hence any repetitive description of similarelements that was already detailed with respect to FIG. 3 is omittedherein for clarity, and rather the differences between FIG. 3 and FIG. 7are detailed. In general, both embodiments of FIGS. 3 and 7 makeparallel coupling decisions based on differences in potential betweeneach battery. The FIG. 3 embodiment does so by directly utilizing Batt_Aand Batt_B voltage signals from each battery.

[0068] In contrast, the selector circuit 714 of FIG. 7 receives I_A andI_B signals representative of the current flow along path 797 and path799 respectively. Path 797 is coupled between Battery A and node 781,and path 799 is coupled between Battery B and the same node 781. Thecurrent flow along paths 797, 797 may represent charging current to eachbattery or discharging current from each battery depending on thesituation.

[0069] Such I_A and I_B signals may be input from the charger circuit722. Alternatively, such I_A and I_B signals may be input directly fromsensors 791, 793 designed to sense current along paths 797, 799respectively. For instance, such sensors 791, 793 may be separate senseresistors. The selector circuit 714 has input terminals 780-1 and 780-2to receive the I_A and I_B signals from any variety of sources. Such I_Aand I_B signals may then be transferred to the controller 715 of theselector circuit 714.

[0070]FIG. 8 illustrates a more detailed block diagram of the controller715 of the selector circuit 714 of FIG. 7. Many elements of FIG. 8 aresimilar to that of FIG. 4 and, as such, are labeled similarly. Hence anyrepetitive description of similar elements is omitted herein for clarityand rather the differences between FIG. 4 and FIG. 8 are detailed. Inparticular, the comparator CMP4 and the parallel battery use circuit 476(together with the associated BATTCOMP and PBUE signals) of FIG. 4 havebeen removed in selector circuit 714.

[0071] Instead, the selector circuit 714 receives the I_A and I_Bsignals at input terminals 780-1 and 780-2 as previously detailed andmay then provide such signals to the selector output circuit 870 of thecontroller 715. The selector output circuit 870 compares such I_A andI_B signals with a current threshold level I_TH and makes switchingdecisions for switches SW3, SW4, SW5, and SW6 based on such comparisonsas further detailed herein. The current threshold level I_TH may be thesame for each battery. Alternatively, the current threshold level I_THmay be different for Battery A (I_THA) and for Battery B (I_THB). Thoseskilled in the art will recognize a variety of ways to make such acomparison between the I_A and I_B signals and the current thresholdlevel I_TH or levels I_THA and I_THB. For example, the selector outputcircuit 870 may have one comparator that compares the I_A signal with anI_THA signal for Battery A and another comparator that compares the I_Bsignal an I_THB signal for Battery B.

[0072] The comparisons made by the selector output circuit 870 willprovide a “low” or “high” battery current signal for each battery. A“low” current signal is representative of a current level flowing in theproper direction less than the associated threshold level or flowing ina direction opposite of the expected current flow. Current flowing inthe opposite direction of expected current flow would be current flowinginto the respective battery when the battery is supposed to delivercurrent (in discharge mode) or current flowing from the respectivebattery when the battery is supposed to receive current (in chargemode).

[0073] For example, if Battery A is in discharge mode the expectedcurrent flow direction is from Battery A to the system. A current fromBattery A as indicated by the I_A signal less than an I_TH level wouldprovide a “low” current control signal for Battery A. In addition, acurrent flow to Battery A regardless of its nominal level would alsoprovide a “low” current control signal for Battery A. The chargingcircuit 722 may be able to provide I_A and I_B signals representative ofcurrent magnitude and direction to each battery. In addition, a varietyof sensors 791, 793 known in the art may also be configured to providecurrent magnitude and direction directly to the selector circuit 714.For instance, if the sensors 791, 793 are sense resistors a positivevoltage drop across a sense resistor may reveal a current flow in onedirection while a negative voltage drop may reveal a current flow in theopposite direction.

[0074] The comparisons made by the selector output circuit 870 willprovide a “high” current signal for each battery if the current flow isin the proper direction and greater than the associated I_TH level.

[0075] Once comparisons between current flow to or from each battery andrespective threshold levels are made, the selector output circuit 870sends appropriate command signals to the switch driver network 417. Theswitch driver network 417 is responsive to such command signals to driveswitches SW3, SW4, SW5, and SW60N and OFF as detailed herein withreference to the tables of FIGS. 9 and 10 to provide protection againstinter-battery current flow when batteries are coupled in parallel to thecommon node 781.

[0076] Turning to FIG. 9, in conjunction with FIGS. 7 and 8, a table 900illustrates respective switch states of switches SW1 to SW6 when powerto the system is provided by a DC power source and not the batteries.Therefore, the ACAV signal from comparator CMP1 is high and the selectoroutput circuit 870 instructs the switch driver network 417 to driveswitch SW1 ON and switch SW2 OFF as detailed in every column of thetable 900.

[0077] The CHGEN signal is “high” in every column of the table 900except for the last column 918. As such, not only is the DC sourcepresent but other conditions (the voltage from the DC source >VT2, and aproper input validation signal VINP1 is present) are satisfied toprovide the high CHGEN signal. Therefore, charging is permitted incolumns 902 to 916 of table 900.

[0078] In columns 902 and 904 the USE_A and USE_B signals indicate thePMU's desire to use Battery A. As such, switches SW5 and SW6 to BatteryB are OFF in both instances. Charging current is provided to Battery Avia closed switch SW3 and diode D4 if the Battery A current signal, asprovided by the selector output circuit 870, is “low” and via closedswitches SW3 and SW4 if the Battery A current signal is “high.”

[0079] In columns 906 and 908 the USE_A and USE_B signals indicated thePMU's desire to use Battery B. As such, switches SW3 and SW4 to BatteryA are OFF in both instances. Charging current is provided to Battery Bvia closed switch SW5 and diode D6 if the Battery B current signal is“low” and via closed switches SW5 and SW6 if the Battery B currentsignal is “high.”

[0080] In columns 910 through 916, the USE_A and USE_B signals indicatethe PMU's desired to couple Battery A and B in parallel (for parallelcharging in this instance). In column 910, the Battery A and Battery Bcurrent signals as provided by the selector output circuit 870 are“low.” In response, the switch driver network 417 drives switch SW3 ON,SW4 OFF, SW5 ON, and SW6 OFF. As such, charging current to Battery A mayflow through closed switch SW3 and diode D4 in parallel with open switchSW4. Similarly, charging current to Battery B may flow through closedswitch SW5 and diode D6 in parallel with open switch SW6. In thisinstance, comparable current could flow to Battery A and B when theirvoltage levels are within a certain close range of one another. If thevoltage levels are not within this close range of one another, anegligible current will flow towards the battery having the highervoltage, e.g., more than about 0.1 volts higher than the other in oneinstance.

[0081] In column 912 the Battery A current signal is “high” and theBattery B current signal is “low.” Such a situation may indicate thatthe potential of Battery B is higher than the potential of Battery A andhence undesirable inter current flow is flowing from Battery B toBattery A. Since Battery B may be providing inter current to Battery A,the net current level to Battery B may be reduced below the thresholdcurrent level I_TH resulting in the “low” Battery B current signal.Advantageously, the selector circuit 714 is configured to open switchSW6 and close switch SW5 in this instance. Diode D6 is in reverse biasto Battery B thereby preventing undesirable inter current flow fromBattery B to Battery A in this instance.

[0082] In column 914 the Battery A current signal is “low” and theBattery B current signal is “high.” Accordingly, the selector circuitopens switch SW4 and closes switch SW3. Therefore, Battery A in thisinstance is prevented from providing inter current flow to Battery B bydiode D4 in reverse bias to Battery A.

[0083] Column 916 represents a normal battery charge mode where bothBattery A and B current signals are “high.” The selector circuittherefore closes switches SW3, SW4, SW5, and SW6 to enable parallelcharging of Batteries in this instance. The “high” Battery A and Bcontrol signals infers that the voltage levels of Battery A and B arewithin acceptable limits of one another hence no need to prevent interbattery flow is necessary. Of course, once the current flow to any onebattery becomes to low the appropriate switches will open as in columns912 and 914 to prevent cross conduction from the higher potentialbattery to the lower potential battery.

[0084] Turning to FIG. 10, in conjunction with FIGS. 7 and 8, a table1000 illustrates respective switch states of switches SW1 to SW6 whensome combination of batteries supplies power to the system.

[0085] In columns 1010 through 1016, the USE_A and USE_B signalsindicate the PMU's desired to couple Battery A and B in parallel (forparallel discharging in this instance). In column 1010, the Battery Aand Battery B current signals as provided by the selector output circuit870 are both “low.” In response, the switch driver network 417 drivesswitch SW3 OFF, SW4 ON, SW5 OFF, and SW6 ON. As such, this is a type ofbattery supply diode mode and the battery A or B with the higher voltagelevel will supply power to the system via diode D3 or D5 in thisinstance.

[0086] In column 1012 the Battery A current signal is “high” and theBattery B current signal is “low.” Such a situation may indicate thatthe potential of Battery A is higher than the potential of Battery B andhence undesirable inter current flow is flowing from Battery A toBattery B. Since Battery A may be provided inter current to Battery B,the net current level from Battery B in this discharge mode may bereduced below the threshold current level I_TH resulting in the “low”Battery B current signal. Advantageously, the selector circuit 714 isconfigured to open switch SW5 and close switch SW6 in this instance.Diode D5 is in reverse bias to Battery A thereby preventing undesirableinter current flow from Battery A to Battery B in this instance. BatteryB is still able to provide discharge current to the system through diodeD5. However, if the output voltage of Battery B falls below a minimumoutput voltage level to direct bias diode D5, Battery B would then notbe able to supply current to the system and the entire supply current tothe system would be provided by Battery A.

[0087] In column 1014 the Battery A current signal is “low” and theBattery B current signal is “high.” Accordingly, the selector circuitopens switch SW3 and closes switch SW4. Diode D3 is in reverse bias toBattery B thereby preventing undesirable inter current flow from BatteryB to Battery A in this instance. Battery A is still able to providedischarge current to the system through diode D3. However, if the outputvoltage of Battery A falls below a minimum output voltage level todirect bias diode D3, Battery A would then not be able to supply currentto the system and the entire supply current to the system would beprovided by Battery B.

[0088] Column 1016 represents a normal battery discharge mode where bothBattery A and B current signals are “high.” The selector circuittherefore closes switches SW3, SW4, SW5, and SW6 to enable normalparallel discharging of Batteries A and B in this instance. The “high”Battery A and B control signals infers that the voltage levels ofBattery A and B are within acceptable limits of one another hence noneed to prevent inter battery flow is necessary. Of course, once thecurrent flow to any one battery becomes too low the appropriate switcheswill open as in columns 1012 and 1014 to prevent cross conduction fromthe higher potential battery to the lower potential battery.

[0089] Turning to FIGS. 11A to 11C, yet a further example of the abovedetailed switching scheme of the selector circuit 714 is illustratedwhere Battery A and B are in a battery discharge mode. FIG. 11Aillustrates normal parallel discharge operation of Batteries A and B.Battery A supplies current Ia along path 1197 via closed switches SW3and SW4 and Battery B supplies current Ib to the system via closedswitches SW5 and SW6 along path 1199. The Ia and Ib currents sum at node1181 to provide a system current equal to the sum of Ia and Ib. As longas current Ia and Ib remain above respective threshold current levels,the selector circuit 714 maintains switches SW3, SW4, SW5, and SW6 ON asdetailed in column 1016 of FIG. 10.

[0090]FIG. 11B represents an unacceptable cross conduction state whereBattery B provides a current Ia to Battery A. This may occur if BatteryA discharges much faster than Battery B. If all the switches SW3, SW4,SW5, and SW6 were to remain ON, gradually the current provided byBattery A would decrease. In addition, at a certain point in time, partof the current supplied by Battery B would be diverted and flow towardsBattery A and eventually the net current would be to Battery A asopposed to from Battery A.

[0091]FIG. 11C illustrates how the internal logic of the selectorcircuit 714 avoids the undesirable case of FIG. 11B. The selector outputcircuit 870 of the selector circuit 714 drives switch SW3 OFF if thedischarging current of Battery A falls below its associated thresholddischarge current level (see column 1014 of FIG. 10). Therefore, BatteryA is still able to supply current to the system through closed switchSW4 and diode D3 in parallel with open switch SW3. Advantageously, diodeD3 is reversed bias with respect to Battery B to prevent crossconduction from Battery B to Battery A. In addition, if the outputvoltage of Battery A then falls below a minimum output voltage level todirect bias diode D3, Battery A would then not be able to supply currentto the system and the entire supply current to the system would beprovided by Battery B.

[0092] The embodiments that have been described herein, however, are butsome of the several which utilize this invention and are set forth hereby way of illustration but not of limitation. It is obvious that manyother embodiments, which will be readily apparent to those skilled inthe art, may be made without departing materially from the spirit andscope of the invention as defined in the appended claims.

What is claimed is:
 1. A power supply system comprising: a first pathconfigured to be coupled to a first battery; a second path configured tobe coupled to a second battery, said first path and said second pathcoupled to a common node; a first switch and a second switch coupled tosaid first path and configured to allow selective coupling of said firstbattery to a load for discharging said first battery; a third switch anda fourth switch coupled to said second path and configured to allowselective coupling of said second battery to said load for dischargingsaid second battery; and a selector circuit configured to close saidfirst, second, third, and fourth switch to allow discharging of saidfirst battery and said second battery in parallel if a first dischargecurrent level from said first battery is greater than a first dischargethreshold level and a second discharge current level from said secondbattery is greater than a second discharge threshold level.
 2. The powersupply system of claim 1, wherein said first discharge threshold levelis equal to said second discharge threshold level.
 3. The power supplysystem of claim 1, wherein said first switch and second switch arefurther configured to allow selective coupling of said first battery toa DC power source for charging said first battery; wherein said thirdswitch and said fourth switch are further configured to allow selectivecoupling of said second battery to said DC power source for chargingsaid second battery; and wherein said selector circuit is furtherconfigured to close said first, second, third, and fourth switch toallow charging of said first battery and said second battery in parallelif a first charge current level to said first battery from said DC powersource is greater than a first charge threshold level and a secondcharge current level to said second battery from said DC power source isgreater than a second charge threshold level.
 4. The power supply systemof claim 3, wherein said first charge threshold level is equal to saidsecond charge threshold level.
 5. The power supply system of claim 1,further comprising a first diode in parallel with said first switch, asecond diode in parallel with said second switch, a third diode inparallel with said third switch, and a fourth diode in parallel withsaid fourth switch.
 6. The power supply system of claim 3, furthercomprising a first diode in parallel with said first switch, a seconddiode in parallel with said second switch, a third diode in parallelwith said third switch, a fourth diode in parallel with said fourthswitch.
 7. The power supply system of claim 5, wherein said first diodeis in reverse bias with said second battery and said selector circuit isfurther configured to open said first switch and close said secondswitch if said first discharge current level is less than said firstdischarge threshold level.
 8. The power supply system of claim 6,wherein said second diode is in reverse bias with said first battery andsaid selector circuit is further configured to open said second switchand close said first switch if said first charge current level is lessthan said first charge threshold level.
 9. The power supply system ofclaim 7, wherein said third diode is in reverse bias with said firstbattery and said selector circuit is further configured to open saidthird switch and close said fourth switch if said second dischargecurrent level is less than said second discharge threshold level. 10.The power supply system of claim 8, wherein said fourth diode is inreverse bias with said second battery and said selector circuit isfurther configured to open said fourth switch and close said thirdswitch if said second charge current level is less than said secondcharge threshold level.
 11. An electronic device comprising: a firstpath configured to be coupled to a first battery; a second pathconfigured to be coupled to a second battery, said first path and saidsecond path coupled to a common node; a first switch and a second switchcoupled to said first path and configured to allow selective coupling ofsaid first battery to a system of said electronic device for dischargingsaid first battery; a third switch and a fourth switch coupled to saidsecond path and configured to allow selective coupling of said secondbattery to said system for discharging said second battery; and aselector circuit configured to close said first, second, third, andfourth switch to allow discharging of said first battery and said secondbattery in parallel if a first discharge current level from said firstbattery is greater than a first discharge threshold level and a seconddischarge current level from said second battery is greater than asecond discharge threshold level.
 12. The electronic device of claim 11,wherein said first discharge threshold level is equal to said seconddischarge threshold level.
 13. The electronic device of claim 11,wherein said first switch and second switch are further configured toallow selective coupling of said first battery to a DC power source forcharging said first battery; wherein said third switch and said fourthswitch are further configured to allow selective coupling of said secondbattery to said DC power source for charging said second battery; andwherein said selector circuit is further configured to close said first,second, third, and fourth switch to allow charging of said first batteryand said second battery in parallel if a first charge current level tosaid first battery from said DC power source is greater than a firstcharge threshold level and a second charge current level to said secondbattery from said DC power source is greater than a second chargethreshold level.
 14. The electronic device of claim 13, wherein saidfirst charge threshold level is equal to said second charge thresholdlevel.
 15. The electronic device of claim 11, further comprising a firstdiode in parallel with said first switch, a second diode in parallelwith said second switch, a third diode in parallel with said thirdswitch, and a fourth diode in parallel with said fourth switch.
 16. Theelectronic device of claim 13, further comprising a first diode inparallel with said first switch, a second diode in parallel with saidsecond switch, a third diode in parallel with said third switch, afourth diode in parallel with said fourth switch.
 17. The electronicdevice of claim 15, wherein said first diode is in reverse bias withsaid second battery and said selector circuit is further configured toopen said first switch and close said second switch if said firstdischarge current level is less than said first discharge thresholdlevel.
 18. The electronic device of claim 16, wherein said second diodeis in reverse bias with said first battery and said selector circuit isfurther configured to open said second switch and close said firstswitch if said first charge current level is less than said first chargethreshold level.
 19. The electronic device of claim 17, wherein saidthird diode is in reverse bias with said first battery and said selectorcircuit is further configured to open said third switch and close saidfourth switch if said second discharge current level is less than saidsecond discharge threshold level.
 20. The electronic device of claim 18,wherein said fourth diode is in reverse bias with said second batteryand said selector circuit is further configured to open said fourthswitch and close said third switch if said second charge current levelis less than said second charge threshold level.
 21. A method ofensuring safe operation of batteries in parallel, said methodcomprising: receiving a control signal from an associated powermanagement unit representative of a desired parallel coupling of atleast a first battery and a second battery to a common node; receiving afirst current signal representative of a first current level along afirst path coupled between said first battery and said common node;receiving a second current signal representative of a second currentlevel along a second path coupled between said second battery and saidcommon node; comparing said first current signal to a first thresholdcurrent level and said second current signal to a second thresholdcurrent level; and coupling said first battery and second battery inparallel to said common node if said first current signal is greaterthan said first threshold current level and said second current signalis greater than said second threshold current level.
 22. The method ofclaim 21, further comprising: preventing discharge current flow fromsaid first battery to said second battery if said second current signalis less than said second threshold current level.
 23. The method ofclaim 21, further comprising: preventing charging current flow from saidfirst battery to said second battery if said first current signal isless than said first threshold current level.